Inverters are operated to convert direct current (DC) into an alternating current (AC) which is suitable to be fed into an energy supply grid and which complies with grid standards. Such inverters may be used in photovoltaic (PV) installations. If galvanic separation between the energy supply grid and the inverter is desired or required due to guidelines or legal provisions for certain energy supply grids, a transformer is arranged between an AC output of the inverter and the energy supply grid.
For large PV installations which are directly connected to a medium voltage grid, the use of a transformer is essential for voltage adjustment. Additionally, it is usually required that the PV installation is effectively grounded. The requirement for effective grounding may be implemented in a technically proper and cost effective way by using a so-called Ynd transformer since such Ynd transformer does not comprise an electrically conductive path for possible fault currents running from the medium voltage side of the transformer into the PV installation. In some cases the use of such Ynd transformer is explicitly specified by the operator of the respective energy supply grid.
Current guidelines and regulations may further require that faults occurring in the energy supply grid can be detected from within the PV installation and that the PV installation disconnects itself from the energy supply grid after having detected certain types of faults in the energy supply grid. A relevant fault in this regard is given by a single-phase phase loss, i.e. an interruption of one of usually three phases occurring between the PV installation and the energy supply grid or within the energy supply grid, respectively.
For feeding-in of electrical energy into the energy supply grid, power switches of an output stage, e.g. of an output bridge of the inverter are actuated using a pulse-width modulation (PWM) method. Output currents and output voltages and their respective fundamental frequency are measured. Control values for the output currents are determined based on differences between target values and actual values of the output currents. Alternatively, in so-called voltage-controlled inverters, differences between target values and actual values of the output voltages are calculated and target values for the output voltages are determined. Pulse-width modulated actuation signals for the power switches of the inverter are generated based on the target values for the output currents or the output voltages, respectively, and the power switches are actuated according to the actuation signals.
US 2006/0004531 A1 discloses a method in which a fault within an energy supply grid is detected based on values of currents and voltages which are measured at a point where energy is feed into the energy supply grid. These current and voltage values are fed into a control loop of an inverter which is connected to the energy supply grid and evaluated in order to detect a fault.
In this method, the inverter is coupled to the energy supply grid directly, i.e. without interposition of a transformer.
If the electrical energy is fed-in via a transformer arranged between the inverter and the energy supply grid, certain faults within the energy supply grid may be projected onto the AC grid on the inverter-side of the transformer, i.e. onto the low-voltage side of the transformer. Hence, such faults may be detected on the low-voltage side of the transformer and taken into account if applicable. This projection strongly depends on the type of fault and on the type of transformer. Regarding the Ynd transformers, single-phase phase losses within the energy supply grid are not projected onto the AC grid beyond the transformer, i.e. within the PV installation. In such cases, according to the state of the art, a detection of the single-phase phase loss is only possible on the high voltage side of the transformer which is disadvantageous due to far more complex measurement equipment required as compared to measurements on the low voltage side of the transformer. When using other types of transformers, e.g. so-called YNy transformers, a phase loss is projected onto the PV installation side of the transformer and may be detected and taken into account using known operating methods of the inverter. On the other hand, an YNy transformer does not provide effective grounding which may be required as mentioned above.